Dental arch appliances

ABSTRACT

An intra-oral appliance that is associated with a pair of dental arches and has an attaching strap extending from a mesial region thereof for attachment to an article that is worn by the user of the appliance, characterized in that the attaching strap includes multiple layers.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of applicant's pendingapplication Ser. No. 07/801,673 filed 02 Dec. 1991, now U.S. Pat. No.5,406,962 and Ser. No. 08/024,854 filed 26 Feb. 1993, now U.S. Pat. No.5,406,963 and a continuation of Ser. No. 07/780,545 filed 22 Oct. 1991,now abandoned, which is a continuation of Ser. No. 07/616,329 filed 21Nov. 1990, now abandoned, which is a continuation-in-part of Ser. No.07/329,407 filed 27 Mar. 1989, now abandoned.

FIELD OF THE INVENTION

This invention relates to appliances that are used in association withdental arches, such as mouthguards, splints, and impression trays.

BACKGROUND AND SUMMARY OF THE INVENTION

The improvements that are the subject of this patent application are theresult of Applicant's continuing work on inventions disclosed inprevious patent applications in his name. Applicant's issued patentsare: U.S. Pat. Nos. 4,955,393 dated Sep. 11, 1990; and 4,983,334 datedJan. 8, 1991. Pending applications are: Ser. Nos. 07/469,286 filed Jan.24, 1990; and 07/780,545 filed Oct. 22, 1991, which has the benefit ofSer. No. 07/616,329 filed Nov. 21, 1990 and of Ser. No. 07/329,407 filedMar. 27, 1989.

The references cited in those applications constitute art that is knownto Applicant, and a recently issued patent on the subject of mouthguardsis U.S. Pat. No. 5,031,638 issued Jul. 16, 1991.

In Applicant's prior patent applications on the subject matter hereof,new and unique multi-layer (multi-laminar) mouthguard constructions aredisclosed. Such multi-layer constructions provide significantimprovement in mouthguard performance and features. In one aspect, thepresent invention relates to further improvements in multi-layermouthguards, both as to the multi-layer structure thereof and to methodsfor making them. In another aspect, the invention relates to new andunique molding methods for fabricating mouthguards, particularly to:supporting a main body of a mouthguard within a cavity of a mold while aliner is molded onto the main body; and building up a multi-layermouthguard one layer at a time by a process that the Applicant refers toas "pyramid molding". A further aspect of the invention relates tomouthguards that have an attaching strap for attachment, or tethering,to an article worn by the user of the mouthguard. Such an article may bea sports headgear, i.e. the face mask or face bar of a football helmetfor example. A still further aspect of the invention relates to a lipprotector that fits onto such an attaching strap and over the lips ofthe user.

Principles of the invention are not necessarily limited to a mouthguardas that word is commonly understood. Principles are applicable tointra-oral appliances associated with at least one dental arch,including impression trays for taking an impression of a dental arch andorthodontic splints.

Further aspects of the invention, along with other features, advantagesand benefits will be seen in the ensuing description and claims whichare accompanied by drawings. The drawings describe a presently preferredembodiment of the invention according to the best mode contemplated atthe present time for carrying out the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transverse cross sectional view through a first layer of amouthguard.

FIG. 2 is a transverse cross sectional view through a first layer ofanother embodiment of mouthguard.

FIGS. 3-12 are respective transverse cross sectional views through tendifferent embodiments of mouthguard showing different second layers inassociation with the first layer of FIG. 1.

FIGS. 13-22 are respective transverse cross sectional views through theembodiments of FIGS. 3-12 respectively showing the addition ofrespective third layers.

FIG. 23 is a view like that of FIG. 22, but with a different form ofthird layer.

FIGS. 24-34 are respective transverse cross sectional views through theembodiments of FIGS. 13-23 respectively showing the addition ofrespective fourth layers.

FIG. 35 is a plan view of a somewhat schematic nature illustrating amolding apparatus for making the mouthguards represented by FIGS. 1-34.

FIG. 36 is a plan view of another embodiment of mouthguard.

FIG. 37 is a transverse cross sectional view in the direction of arrows37--37 in FIG. 36.

FIG. 38 is a fragmentary side elevational view of a mold for making themouthguard of FIGS. 36 and 37.

FIG. 39 is a side elevational view of a portion of a mold for making amouthguard like that of FIG. 36.

FIG. 40 is a plan view of an attaching strap extending from amouthguard.

FIG. 41 is a fragmentary view of a portion of the strap including amodification in accordance with the invention.

FIG. 42 is a fragmentary perspective view of a portion of FIG. 41.

FIGS. 43-45 are fragmentary elevational views of other embodiments ofattaching straps.

FIG. 46 is a front elevational view of a lip guard in accordance withthe invention.

FIG. 47 is a rear view of FIG. 46.

FIG. 48 is a cross sectional view in the direction of arrows 48--48 inFIG. 46.

FIG. 49 is a view of one of the parts of FIG. 48 by itself.

FIG. 50 is a view like FIG. 48 illustrating usage.

FIG. 51 is a transverse cross section of a splint.

FIG. 52 is a plan view of the inside of one half of a mold forfabricating an intra-oral device according to the present invention.

FIG. 53 is a plan view of the inside of the other half of the mold.

FIG. 54 is an enlarged transverse cross section view through the twomold halves in closed condition for molding an intra-oral device, astaken along line 54--54 in FIG. 52.

FIG. 55 is the same type of view as FIG. 54, but along line 55--55 inFIG. 52.

FIG. 56 is a plan view of a portion of an intra-oral device shown with afeature for locating said portion in a mold.

FIG. 57 is a side elevation view of FIG. 56 showing said feature.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1 and 2 illustrate two examples of a first layer 60 for amouthguard. Layer 60 is fabricated by molding a suitable moldingmaterial into the illustrated cross sectional shape. Preferably suchmolding is conducted in the cavity of a mold shaped to provide thegeneral U-shape that is characteristic of a mouthguard. The preferredmolding procedure is one in which the flowable material, for exampleethylene vinyl acetate (EVA), is injected under pressure into the cavityand allowed to cure. Layer 60 forms that portion of the mouthguard thatis associated with the upper dental arch. It comprises a trough 62defined by an occlusal wall 64, a lingual wall 66, and a labial wall 68.The tip end of the labial wall is rounded at 70, and the labial wallalso contains a large groove 72 as shown. The only difference betweenthe embodiments of FIGS. 1 and 2 is that in FIG. 2 the surface ofocclusal wall 64 contains two spaced apart ridges 74 opposite trough 62.

FIGS. 3-12 illustrate ten different embodiments having a second layer 76united with the FIG. 1 embodiment of first layer 60. Each of the secondlayers is fabricated by molding a suitable molding material into theillustrated cross sectional shape. A portion of the mold in which thefirst layer 60 was fabricated may be used to retain the first layer andbe cooperatively associated with further mold structure to cooperativelydefine a further mold cavity having a portion that is not occupied byfirst layer 60 so as to define a volume having the shape of the secondlayer 76. Flowable material is injected into this further mold cavityand allowed to cure to form the second layer 76 united to layer 60. Suchunison may be obtained by the method described in the above-referencedapplication Ser. No. 07/780,545 wherein the second layer directly bondsto the first layer. The second layer is preferably a material which issomewhat harder (higher durometer) than layer 60. It may for example bea higher durometer EVA. In all embodiments of FIGS. 3-12, the secondlayer 76 is united to a surface of first layer 60 that is oppositetrough 62. In the embodiments of FIGS. 3-9, the second layer 76 coverswalls 64, 66, and 68; in FIG. 10, only wall 64; in FIG. 11, only wall68; and in FIG. 12, only wall 66. Alternatively, it would be possible tocarry out the molding of the second layer without the use of a part ofthe mold that was used to fabricate the first layer, but this wouldrequire that the first layer be removed from the mold in which it wasfabricated and placed in a potion of the cavity of the second mold,rather than being retained in a portion of the mold in which it wasmolded and which is also used for cooperative association with thefurther mold structure used to fabricate the second layer 76.

FIGS. 13-22 illustrate the ten embodiments of FIGS. 3-12 having receiveda third layer 78. The molding of the third layer is conducted inanalogous fashion to the molding of the second layer. The united firstand second layers are retained in a portion of the mold used to createthe first layer, which is then cooperatively associated with a stillfurther mold structure to cooperatively define a still further cavityhaving a portion that is not occupied by the first and second layers soas to define a volume having the shape of the third layer. Suitablemolding material is flowed into the unoccupied volume and allowed tocure so as to unite the third layer with the previously united first andsecond layers, and thereby form the shapes as shown in FIGS. 13-22.Preferably the material of the third layer has a higher durometer thanthat of the second layer, and for example a suitable material may be ahigher durometer EVA. As was done in fabricating the second layer, thethird layer may be directly molded to the underlying layer(s) coveredthereby. As can be seen from the various drawing Figs., the third layeris in some instances applied only to the first layer, in other instancesonly to the second layer, and in still other instances to both the firstand second layers. FIG. 23 is the only instance illustrated where thethird layer is applied to only the first layer, and in this embodimentthe second and third layers are used to provide the mouthguard withdifferent durometer labial and lingual wall portions. In all embodimentsof FIGS. 13-23, there is a shallow trough 80 provided opposite trough62.

The finished transverse cross sectional shape for the eleven embodimentsof mouthguards represented by FIGS. 24-34 comprises a layer of material82 in each trough 80. Layer 82 is the fourth layer and is fabricated inthe same manner as the second and third layers. The united first,second, and third layers are retained in a portion of the mold used tofabricate the first layer, and they are then cooperatively associatedwith yet another mold structure to cooperatively define yet another moldcavity having a portion that is not occupied by the first, second, orthird layers so as to define a volume having the shape of the fourthlayer. Suitable molding material, which once again may be EVA, is flowedinto this volume and allowed to cure so as to unite with the previouslyunited first, second, and third layers. Preferably the fourth layer hasa lower durometer than the second and third layers. Trough 80 and layer82 are intended to receive the lower dental arch of the user. As can beseen in FIGS. 24-34, layer 82 covers only the third layer in someinstances, only the second and third layers in others, and in theembodiment of FIG. 26, it unites with all three layers.

FIG. 35 depicts a molding apparatus 84 at which the four-layermouthguards are fabricated. The apparatus comprises a rotary table 86that can be indexed about an axis of rotation 88. Four injectors 90, 92,94, and 96 are stationarily arranged at ninety degree increments aroundthe perimeter of table 86. The mold that is used to fabricate the firstlayer 60 is at station #1 where it is associated with injector 90; themold that is used to fabricate the second layer 76 is at station #2where it is associated with injector 92; the mold that is used tofabricate the third layer 78 is at station #3 where it is associatedwith injector 94; and the mold that is used to fabricate the fourthlayer 82 is at station #4 where it is associated with injector 96. Thusthe first layer of a first mouthguard, the second layer of a secondmouthguard, the third layer of a third mouthguard, and the fourth layerof a fourth mouthguard are simultaneously fabricated. Thereafter thetable is indexed clockwise forty-five degrees to place eachmouthguard-in-process at a cooling station where the previously injectedlayer is allowed to at least partially cure. The cooling stationimmediately after the fourth injection station is the one at which thefinished mouthguard is removed.

At each station, the mold that contains the mouthguard-in-process isassociated with the corresponding injector for introducing moldingmaterial into the mold cavity. After the material has been injected, theinjector is disconnected from the mold and the table is indexed, firstto the cooling stations and then to the next injection station. Asexplained earlier, a mouthguard-in-process on the table is carried by aportion of the mold used to make the first layer. This portionconstitutes a carrier for the mouthguard-in-process that is indexed frominjector to injector by the table. At each molding station this carrieris cooperatively associated with particular mold structure unique tothat station for creating the particular layer that is to be molded atthat station. Thus, the particular mold structure unique to each stationremains at that station and does not index with the table. To the extentthat molding material is introduced into a cavity via the carrier, it isnecessary for the injector to be disconnected after the injection, asmentioned above. However, to the extent that material is introduced intoa cavity through the particular mold structure that remains at astation, then the injector does not necessarily have to be disconnectedfrom that structure. At each cooling station, it is possible thatcooling equipment could be connected to a carrier upon arrival anddisconnected before departure.

FIGS. 36-39 relate to an embodiment of mouthguard which comprises a mainbody 90' onto which liner material 92' is molded. The mouthguard has ageneral U-shape comprising an upper trough 94' for an upper dental archand a lower trough 96' for a lower dental arch. Liner material linesboth troughs, and the occlusal wall 98 of the main body 90' may comprisea series of holes 100 that provide for the upper and lower liners to beintegrally joined.

Main body 90' is a material such as EVA, and therefore when it is placedinto a mold cavity for the purpose of injection molding liner materialonto it, it must be properly located and supported therein. It has beendiscovered that suitable support can be obtained by providing the moldcavity with a series of small stand-off pins that engage the occlusalwall of the main body and with wall-engaging members that engage lingualand labial walls of the main body. Such features may be usedindividually or jointly in any given mold. FIG. 38 shows a moldcomprising separable mold members in separated condition and designated102, 104 respectively. When they are operated closed to support a mainbody 90' within the cavity which they cooperatively define, a series ofstand-off pins 106 engage the lower surface of occlusal wall 98 while aseries of stand-off pins 108 engage the upper surface of the occlusalwall. The stand-off pins are arranged in a pattern around the main bodyas can be appreciated from consideration of FIG. 36 which shows aresulting pattern of small holes 110 that appear in the liner portionscovering occlusal wall 98 after the liner has been injected into thecavity. The injection of the liner material into the cavity fills avolume that is unoccupied by the main body and defines the shape for theliner. The injected liner material covers the occlusal wall except atthe locations where the occlusal wall surface is engaged by thestand-off pins. The holes 110 extend through the liner to the main body,but they have no significant effect on the mouthguard because they aremuch smaller in transverse cross section than any tooth of an arch withwhich the finished mouthguard is intended to be used.

FIG. 39 depicts the mold to comprise a first set of wall-engagingmembers 112, 114 and a second set of wall-engaging members 116, 118. Themembers 112, 114 are arranged in opposition for movement toward eachother to grip the labial wall 120 of the main body while members 116,118 are arranged for movement away from each other to grip the lingualwall 122 of the main body. Members 112, 114 have concave faces contouredto match the convex shapes of the portions of the labial wall engagedthereby. When a main body is loaded into the open mold members, themembers 112, 114 are retracted, as are members 116, 118. The members112, 114 are advanced to grip the main body and the members 116, 118 arealso advanced to grip it. The motions for operating the members may begenerated by cam mechanisms that couple the motion of the closing moldmembers to the members 112, 114, 116, 118. Members 112, 114 have amesio-distal extent that is greater than that of members 116, 118.Alternatively to the use of cam mechanisms for operating the members112, 114, 116, 118, as described, they may be operated by poweredslides, cylinder actuated slides for example.

A still further feature of the invention relates to an attaching, ortethering, strap that is sometimes used with sports mouthguards. FIG. 40shows such a strap 124 extending from the mesial region of the U-shapedmouthguard 126. The free end of strap 124 comprises a ball tip 128 atthe end of a taper 130. The strap has uniform thickness throughout itslength to point of merger with a protuberance 132 of the U-shapedmouthguard. The strap also has several elongated through-holes 134, 136,138. Tethering of the mouthguard to a sports helmet facebar for exampleis accomplished by looping the far end of the strap around the bar andsnapping the ball tip 128 through a selected one of the holes 134, 136,138. Between the taper 130 and the protuberance 132, strap 124 has auniform width. Accordingly, the application of a tensile force along thelength of the strap will tend to ultimately sever the strap at one ofthe regions designated 140 if the tensile force becomes sufficientlylarge. This is because the locations of the holes are the weakestlocations along the length of the strap. When a tethered mouthguard isin use, relative movement of the point of tethering away from theU-shaped mouthguard in the user's mouth will generate tensile force inthe strap. The Applicant has discovered that a break-away feature in thestrap may be useful when the tensile force exceeds a certain magnitude.FIGS. 41-45 illustrate several embodiments for such a break-awayfeature.

FIGS. 41 and 42 show the incorporation of the feature by providing slits142 on opposite sides of the elongated throughhole 140 nearestprotuberance 132. The slits are short but provide a certain weakeningthat will cause the strap to break across hole 138 in response to theapplication of a predetermined magnitude of tensile force to be strapbetween the point of tethering and the mouthguard.

FIG. 43 shows a weakening 144 that will provide the breakaway feature.The strap in this embodiment is of multi-layer construction, suchconstruction per se being believed broadly new in a mouthguard tetheringstrap, and comprises transverse discontinuities in upper and lowerlayers 146, 148 adjacent the mouthguard. The strap has a central layer150 between the layers 146, 148. Each of the layers 146, 148, 150 can bemolded integrally with a corresponding layer of the mouthguard.

FIG. 44 shows a line of weakening 152 that provides the break-awayfeature in the form of a V-notch extending transversely across the strapin one surface thereof. This V-notch can be molded into the strap.

FIG. 45 depicts the break-away feature as a barbed end 154 of the strapthat is inserted into a complementary barbed socket 156 in themouthguard. Such socket may be in a protuberance such as 132, or it maybe formed in the relatively harder main body 90 if the protuberance 132is formed in relatively less rigid material such as that lining the mainbody.

FIGS. 46-50 relate to a further aspect of the invention, namely a lipprotector 158 that fits onto a tethering strap of a mouthguard. Lipprotector 158 is fabricated by multi-layer molding. It comprises a mainbody 160 of relatively harder material and a liner 162 of relativelysofter material. Main body 160 has a generally concave shape forconformance with the lips and surrounding area. The rear face contains acavity 164 that contains liner 162. A through-hole 166 extends centrallythrough both main body and liner to allow the lip guard to fit onto astrap 124 in the manner of FIG. 50. In that Fig. it can be seen that theliner protrudes rearwardly for contacting the lips. In the perimeter ofthe main body 160 there are several airholes 168.

FIG. 51 shows the transverse section of a U-shape device in the form ofa splint 170. Splint 170 is fabricated according to the method describedabove, but comprises only three layers 172, 174, 176. Layer 174constitutes the main body which has troughs on opposite sides. Layer 172is softer than layer 174 and lines the trough that receives the upperdental arch. Layer 176 is harder than the layer 174 and fills the troughthat is associated with the lower dental arch. Layer 172 provides forthe impression of an exact impression of the upper arch. The surface oflayer 176 is flat and sufficiently hard that the teeth of the lower archcan move over it. Thus the device of FIG. 51 is a splint that is usefulin the treatment of TMJ disorders.

Principles of the invention can be applied to various intra-oralappliances, and are not necessarily limited to mouthguards as that wordis commonly understood by the general public. For example, the device ofFIG. 51 just described may have the appearance of a mouthguard, but itis intended to function as a splint because of the specific nature ofits troughs and the material of the liners thereof. In the case of whatis commonly understood as a mouthguard, the layers disposed in thetroughs are tooth-impressionable so that after the mouthguard has beenfabricated in the molding apparatus, the user is capable of creatingtooth impressions by biting into the material after placing themouthguard in the mouth. Depending upon the nature of thetooth-impression material that lines the troughs, heating of themouthguard, such as by placing it in warm water before intra-oralplacement, may facilitate the taking of the tooth impressions. The samemay be true with an impression tray, which is another form of intra-oralappliance associated with at least one dental arch, embodying principlesof the invention. Certain materials, such as EVA for example, may bemolded directly onto an underlying layer without the use of adhesive,and remain tooth-impressionable after having been so directly bonded tothe underlying layer.

In the case of the lip protector, it is possible to use EVA for both themain body and the liner, with the liner being of lower durometermaterial. In such an example, the liner can be directly bonded to themain body by injection molding.

It is possible to eliminate the resulting holes 110 in the embodiment ofFIGS. 36 and 37 without eliminating the use of the stand-off pins. Thiscan be accomplished by making the stand-off pins retractable while themold remains closed. The pins remain extended for a certain time afterthe injection, and are then retracted by a timer-controlled mechanismbefore the flowable material has lost fluency. Upon retraction of thestand-off pins, the still flowable material will flow to fill the holes110 that would otherwise remain if the pins are allowed to remainextended until the injected material loses fluidity. At the time oftheir retraction, the pressure of the injection has dissipated so thatthe need for the stand-off pins to hold the main body has passed. It mayalso be mentioned that stand-off pins could be used to engage the upperand/or lower lips of the buccal wall 120 for the purpose of providingadditional support to the main body. Such usage is intended to beadditional to the use of pins to support the occlusal wall.

In connection with the break-away attaching strap, it may be noted thatthe strap 124 of FIG. 40 illustrates three more or less conventionalholes. The break-away feature, like that of FIGS. 41 and 42 forinstance, may be embodied by creating a fourth elongated hole with slits142 between hole 138 and protuberance 132. Also, the break-away featureof FIG. 45 may be embodied by providing a socket like 156 in theocclusal wall of a main body so that when the barbed end of the strap isinserted, its presence completes the occlusal wall in the region of thesocket. Liner material lining the troughs still overlies the top andbottom of such a socket. Such a socket would have a top view like theview of FIG. 45. (As described earlier, the view of FIG. 45 is in thesame direction as the views of FIGS. 43 and 44.)

In the option of using a set of stand-off pins to engage the upperand/or lower edges of the labial wall 120, it should have been notedthat those pins may also be retracted by a timer-controlled mechanism toavoid the creation of small holes in the liner material that covers suchedges.

FIGS. 52-55 relate to another apparatus and method for making anintra-oral apparatus, which is illustrated as a mouthguard comprising aU-shaped main body of higher durometer material that has upper and lowertroughs covered by respective layers of lower durometer material. Theapparatus comprises a two-piece mold 200a, 200b, that when in closedcondition as shown in FIGS. 54, 55, define two mold cavities 202a, 202b.The portion of cavity 202a in mold piece 200a is designated 202a1, andthe portion in mold piece 200b, 202a2; the portion of cavity 202b inmold piece 200a is designated 202b1, and the portion in mold piece 200b,202b2. The two-piece mold 200a, 200b is used for molding respectivelayers onto a previously fabricated main body-like main body 90' inFIGS. 36-37. Such a main body comprises an occlusal wall 98 containing aplurality of throughholes 100, a labial wall 120, and a lingual wall122. Occlusal wall 98, in cooperation with portions of walls 120, 122,form an upper trough 94' while occlusal wall 98, in cooperation withother portions of walls 120, 122, form a lower trough 96'.

Mold piece 200a comprises, in cavity 202a1, a plurality of pins,corresponding to pins 108, that, with main body 90' properly positioned,fit closely into holes 100, extending as far as the parting plane Pbetween the mold pieces 200a, 200b, when the mold pieces are in closedposition, as depicted by FIGS. 54-55. The fit of the pins to the holesis such that during molding of a layer onto lower trough 96' theflowable molding material does not substantially intrude between thepins and the holes.

Mold piece 200b comprises, in cavity 202a2, a plurality of pins,corresponding to pins 106, that, with main body 90' properly positioned,abut the lower side of occlusal wall 98. These pins function in the samemanner as pins 106 during the molding process previously describedabove.

Mold piece 200a comprises, in cavity 202b1, a plurality of pins 206 thatfunction with respect to molding cavity 202b, in analogous fashion topins 106, when molding is conducted in cavity 202a. Cavity portion 202b2has no pins.

The method comprises a two step molding process wherein a first step ofmolding is conducted in cavity 202a and a second step in cavity 202b.The first step comprises disposing a main body 90' in proper position inone of the two portions of cavity 202a, and then closing the two pieces200a, 200b to the condition shown by FIG. 54 such that pins 108 fit intoholes 100 and pins 106 abut the lower surface of occlusal wall 98. Thecavity is shaped such that a space 210 is provided contiguous with thelower face of the main body, i.e. with trough 96'. Flowable, curablemolding material is introduced into this space by injection molding toform a layer lining the trough. The molding material is allowed to atleast partially cure before the two mold pieces 200a, 200b are opened.The at least partially cured material adheres to the main body beforethe mold pieces are opened, but may still flow slightly to at leastpartially, or even completely, fill the voids left by removal of pins106 from abutment with the lower surface of occlusal wall 98. Theconstruction of the two-piece mold 200a, 200b is such that the materialinjected to cover the lower trough does not intrude onto the upper faceof the main body, i.e. onto trough 94'.

The second step of the molding process comprises removing the main bodyand the adhered lower trough liner from cavity 202a, and disposing samein cavity 202b, as shown by FIG. 55. The second molding cavity 202b isshaped such that portion 202b2 conforms substantially exactly to theshape of the lower trough liner created by the first step. It is alsoshaped such that a space 212 is provided contiguous the upper face ofmain body 90', i.e. contiguous trough 94', when the two pieces 200a,200b are in the closed condition depicted by FIG. 55. The conformance ofportion 202b to the material lining the lower main body trough providessupport from that side. Pins 206 engage the upper face of occlusal wall98. Then, flowable, curable molding material is injected into space 212to form a layer lining the upper trough. The injected material isallowed to at least partially cure into adherence with the upper troughbefore the two mold pieces are opened. The injected material maythereupon flow slightly, either partially or completely, into the voidsleft by disengagement of pins 206 from the upper face of the occlusalwall of the main body. Thereafter, the mouthguard is removed from themold.

The completed mouthguard comprises upper and lower layers lining therespective troughs of the main body. To the extent that some of theinjected material may have partially entered holes 100 during the firststep, the material injected during the second step may integrally jointherewith by entering holes 100 such that the two layers of injectedmaterial integrally join each other through holes 100. Even if the pins108 completely filled holes 100, integral joining of the two injectedlayers can occur by the second injected material completely filling theholes to unite with the first injected material.

A preferred material for the main body and the injected materials isethylene vinyl acetate (EVA), with the injected layers, or liners,having a durometer of less than about 40, while the main body has adurometer in the range of about 70 to about 90, although the moregeneral principles of the invention are not limited to specificdurometers or materials. Injected material that has compatibility withthe main body (both being EVA in the example of the preferredembodiment) allows the injected material to adhere directly onto themain body without the use of separate adhesive. During the second step,the second injected material does not intrude into any covering relationto the first injected layer.

FIGS. 56 and 57 show the main body provided with a locating feature inthe form of one or more apertured tabs 214 that project from the labialwall in a labial-lingual sense. In the illustrated embodiment, thesetabs project outwardly in the labial direction. These apertured tabsfunction to form locators for locating the main body in the mold, whichis provided with spaces for closely accommodating the tabs and pins thatpass through the apertures in the tabs when the mold is closed. Suchfeatures may be provided in either or both of the cavities. After theyhave served their purpose, they are severed from the main body such thatthe labial wall provides a flush surface where the tabs were located.Preferably, none of the injected material that forms the layers coversthe tabs so that the tabs remain exposed and uncovered by injectedmaterial.

The molding process utilizes a mold comprising the two cavities forpurposes of efficiency so that an upper trough of a mouthguard islayered concurrently with the molding of a layer onto the lower troughof the main body of a succeeding mouthguard.

While a presently preferred embodiment of the invention has beenillustrated and described, it should be appreciated that principles areapplicable to other embodiments that are equivalent to the followingclaims.

What is claimed is:
 1. An intra-oral appliance comprising a generallyU-shaped main body that is associated with a pair of dental arches andan attaching strap extending from a mesial region of said main body toan attaching end for attachment to an article that is worn by a user ofthe appliance, characterized in that said attaching strap comprisesmultiple discrete layers, a first of said strap layers extendsintegrally from said main body to said attaching end, a second of saidstrap layers is disposed on said one strap layer, and said second layercomprises a transverse discontinuity at a location along the length ofsaid first layer between said attaching end and the mesial region of themain body to provide at said location, a zone of yielding whereat thestrap will break in response to a predetermined tensile force applied tothe length of the strap.
 2. An intra-oral appliance as set forth inclaim 1 characterized further in that said second layer extends fromsaid location, along the length of said first layer, and onto a portionof said main body.
 3. A lip guard for fitting onto an attaching strap ofan intra-oral appliance, said lip guard comprising:a main body forexternally covering the lips; a hole through said main body for allowingan attaching strap to pass through the lip guard; said main body havinga cavity that is toward the lips and contains a material softer thansaid main body for engaging the lips; said softer material also having ahole for allowing the attaching strap to pass through the lip guard. 4.A lip guard as set forth in claim 3 including a plurality ofthrough-holes in said main body extending through a margin thatsurrounds said cavity.
 5. A method of making an intra-oral appliancecomprising providing a main body having labial and lingual portions withan integral locating formation means projecting from at least one ofsaid portions in a labial-lingual sense; disposing said main body inproper position in a mold by using said integral locating formationmeans; and molding molding material onto said main body; and thereaftersevering said integral formation means from said main body.
 6. A methodas set forth in claim 5 in which said integral locating formation meanscomprises an apertured tab that is associated with a complementaryshaped formation of the mold for locating said main body in properposition in the mold, said molding step leaves said apertured tabexposed and uncovered by the molding material, and said severing stepcomprises severing said tab substantially where said tab joins said mainbody.
 7. In a method of making an intra-oral appliance that isassociated with upper and lower dental arches and comprises a main bodyhaving respective upper and lower faces for respectively engaging upperand lower dental arches, the improvement which comprises:disposing saidmain body in proper position within a portion of a first molding cavity;introducing flowable molding material into a space of said first cavitythat is not occupied by said main body but that confronts at least aportion of one of said faces of said main body such that the flowablematerial flows into covering relationship with said at least a portionof said one face, but is prevented from flowing into covering relationwith at least a portion of the other face; allowing the introducedflowable material to at least partially cure into adherence to said mainbody; removing said main body and adhered material from said cavity anddisposing same in proper position within a portion of a second moldingcavity; introducing flowable molding material into a space of saidsecond cavity that is contiguous said at least a portion of said otherface such that the flowable material flows into covering relationshipwith said at least a portion of said other face; and allowing theintroduced flowable material to at least partially cure into adherencewith said at least a portion of said other face.
 8. The improvement setforth in claim 7 in which said main body has a U-shape wherein a firstof said faces comprises a trough for the respective dental arch, saidtrough having an occlusal wall at the bottom of said trough, and theflowable material is flowed to form a trough-shaped liner for saidtrough.
 9. The improvement set forth in claim 8 in which pins in therespective molding cavity engage said occlusal wall as flowable materialflows to form said liner.
 10. The improvement set forth in claim 9including providing pins in said first molding cavity, and engaging saidpins with said occlusal wall during the introduction of molding materialinto said first cavity.
 11. The improvement set forth in claim 10including engaging said pins with said occlusal wall from opposite sidesthereof.
 12. The improvement set forth in claim 9 including providingholes in said occlusal wall, and fitting some of said pins into saidholes while abutting said occlusal wall with others of said pins atlocations other than the locations of said holes.
 13. The improvementset forth in claim 7 in which the flowable material introduced into saidsecond molding cavity is prevented from flowing to said one face. 14.The improvement set forth in claim 7 further including providing saidmain body with locating tab means that project in a labial-lingualsense, and severing said tabs from said main body at a time aftermolding material has been adhered to said main body.